Amylopectin esterification

Successful methods entail precipitation of the amylose from solution as an insoluble complex, which is removed by high-speed centrifuging the amylopectin is isolated from the supernatant liquor by precipitation with alcohol or, more satisfactorily, by freeze-drying. (Precipitation with alcohol does not always appear to be quantitative, and the physical form of the product obtained by freeze-drying is more satisfactory for subsequent dissolution and esterification.) The amylose can then be further purified by reprecipitation with the same or, preferably, a different complexing agent. 

The physical form of the component is extremely important for ease of esterification for amylose, the order of reactivity is amylose complex > freeze-dried or freshly precipitated amylose > alcohol-precipitated, ether-dried amylose, whereas freeze-dried amylopectin is more reactive than... 

The acetylation of amylopectin with pyridine and acetic anhydride presents more difficulty, even when using freeze-dried material,26 and the most satisfactory method is that involving prior dispersion in formamide, after which esterification occurs readily at room temperature. 

The properties displayed by a starch triacetate, or by other starch esters are dependent upon the type and molecular weight of the starch or starch fraction used for the esterification. The properties may also be influenced by trace constituents or impurities such as compounds of nitrogen and phosphorus which are often observed in starch esters. While the nature of the nitrogenous constituent is not known, it has been demonstrated that much of the phosphorus of potato starch exists as a natural phosphoric ester of the amylopectin fraction. It is not surprising, therefore, that the phosphorus of potato starch is carried through an esterification and may influence the properties of the starch ester. The exact modifying effects of these components on the properties of starch esters, however, are still to be explained. 

Reaction conditions necessary to carry out the modifications described earlier in this section usually result in some decomposition of amylose and amylopectin, even when simple substitution, addition, or crosslinking are involved. As a rule, graft copolymerization produces derivatives of significantly increased molecular weight. Starch grafting usually entails etherification, acetalation, or esterification of starch with vinyl monomers to introduce a reaction site for the further formation of a copolymeric chain. Such a chain would typically consist of either identical or different vinyl monomers (block polymers), or it may be grafted onto another polymer altogether. 

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